It is known that the shifting point for shifting from one transmission stage to another transmission stage, of an automatic transmission of a vehicle can be influenced by various factors. Among these are, e.g., the position of the gear selection lever, program switches, kick-down switches, transmission fluid temperature (or transmission lubricant temperature), rotational speed of the transmission input, and/or braking signals. Many times, in practice, an automatic transmission will execute a shift to a higher or lower transmission stage in a relatively short time period, namely when the automatic control criteria for such shifting of the transmission are satisfied. The result may be rapid back and forth shifting between two transmission stages, sometimes called “oscillative shifting”.
Such back and forth shifting may occur, e.g. when negotiating mountain roads. At constant load, the traction force of the engine after an up-shift is less than the sum of the forces of resistance to the forward motion of the vehicle, wherewith the acceleration of the vehicle decreases until eventually the absolute speed of the vehicle is decreased. If a limit value is reached at which a down-shift ought to be performed, the transmission is caused to so downshift, into a lower transmission stage. In such a lower transmission stage, the traction force is now greater than the aggregate resistance forces, so that the vehicle can again accelerate. If the driver seeks to accelerate, he will continue driving in this mode until once again a limit value is reached at which up-shifting ought to be performed. Such a back and forth shifting between transmission stages, when carried out by an automatic transmission, is regarded as an unpleasant occurrence, because each shifting process entails disengagement of a coupling and thereby interruption of the tractional force.
In the light of this, the underlying problem of the present invention was to devise a means of controlling the shifting point for selecting the currently most favorable transmission stage, in an automatic transmission of a vehicle, whereby disadvantageous back and forth shifting (oscillative shifting) is avoided or is at least reduced.